Washington DC – Ukrainian armed forces have received cluster munitions pledged by the United States to Kyiv to boost its slow-moving counter-offensive against the Russian invasion, a senior army official said Thursday.

"We just got them, we haven't used them yet, but they can radically change (the battlefield)," Ukrainian army commander Oleksandr Tarnavskyi told CNN in an interview.

The United States last week said it would deliver cluster bombs to Ukraine, despite concerns over the long-term risk posed to civilians.

"The enemy also understands that with getting this ammunition, we will have an advantage," Tarnavskyi said.

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Tarnavskyi told CNN Ukrainian forces would not deploy the weapon in heavily populated areas.

"The Russians think that we will use it on all areas of the front... This is very wrong," he said.

President Joe Biden said the decision was "very difficult" but stressed Ukraine needed extra ammunition to refill its depleted stocks.

The controversial weapons can disperse up to several hundred small explosive charges, which can remain unexploded in the ground, creating a risk for civilians after a conflict is over.

They are banned by numerous countries – notably in Europe – who are signatories to a 2008 Oslo Convention, to which neither Russia, the United States or Ukraine are parties.

The Kremlin has said it will take "countermeasures" if the weapon is deployed by Ukraine against its troops.